High resolution wafer inspection system

ABSTRACT

A method for inspecting a region, including irradiating the region via an optical system with a pump beam at a pump wavelength. A probe beam at a probe wavelength irradiates the region so as to generate returning probe beam radiation from the region. The beams are scanned across the region at a scan rate. A detector receives the returning probe radiation, and forms an image of the region that corresponds to a resolution better than pump and probe Abbe limits of the optical system. Roles of the pump and probe beams may be alternated, and a modulation frequency of the pump beam may be changed, to produce more information. Information extracted from the probe signal can also differentiate between different materials on the region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No. 11/952,010filed 6 Dec. 2007 now U.S. Pat. No. 7,714,999 which claims the benefitof U.S. Provisional Patent Applications 60/868,791, 60/868,817,60/868,863, and 60/868,909, filed 6 Dec. 2006, which are incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates generally to inspection systems, andspecifically to wafer inspection systems operating at a high resolution.

BACKGROUND OF THE INVENTION

Inspection of wafers, both for defects in wafer features and to confirmthat the features conform to specified parameters, is an integral partof the wafer fabrication process. One of the methods known in the artfor performing such an inspection uses an opto-thermal pump/probetechnique. In this technique a first optical source (the pump) heats alocation of the wafer, and a second optical source (the probe)investigates the effect of the heating to determine a property of thelocation, such as its resistance. U.S. Pat. No. 6,971,791, to Borden etal., whose disclosure is incorporated herein by reference, describes amethod for identifying defects in a conductive structure of a wafer. Themethod is based on measurements of the heat transfer through theconductive structure.

U.S. Pat. No. 7,136,163, to Borden et al., whose disclosure isincorporated herein by reference, describes evaluation of asemiconductor wafer having two regions with different dopantconcentration profiles. The evaluation performs measurements indicativeof a difference in reflectivity between the regions.

U.S. Pat. No. 7,133,128, to Clarysse, et al., whose disclosure isincorporated herein by reference, describes determining the dopantprofile of doped regions in a semiconductor substrate by using a pumplaser to create excess carriers in the semiconductor substrate.

SUMMARY OF THE INVENTION

In an embodiment of the present invention, an inspection apparatusincludes a pump beam generator, typically comprising a first laser,which generates radiation at a pump wavelength and outputs the radiationas a pump beam. The apparatus includes a probe beam generator, typicallycomprising a second laser, which generates radiation at a probewavelength and outputs the radiation as a probe beam. An optical systemfocuses the pump beam and the probe beam to a region of a substrate,typically a semiconductor substrate, so as to irradiate the region. Theoptical system also scans the two beams across the region at a scanrate. The pump beam generates a level of excess carriers in the region,and the intensity of returning probe beam radiation from the region is afunction of the level.

The optical system conveys the returning probe beam radiation to adetector, which forms an image of the region from the returningradiation. The resolution of the image is better than the Abbe limit ofthe optical system at either the pump wavelength or the probe wavelengthand the detector is sampled in a sample time, which, combined with thescan rate, enables the detector to form the image at an image pixel sizecorresponding to the better resolution.

Typically, the pump beam is modulated, and the returning probe beammodulates in response to the pump beam modulation. The detector isconfigured to detect the modulated returning probe beam, thus improvingthe signal to noise ratio of the image. In some embodiments a phaseshift between the returning probe beam and the pump beam is measured.The phase shift may be correlated with a feature material on thesubstrate, and used to identify the feature material composition. Insome embodiments more than one type of feature is present in theirradiated region, and the different types of features may generatedifferent phase shifts. In this case the different features may beidentified in the image produced, typically by using different colors inthe image for the different types.

In an alternative embodiment of the present invention, a metal iscoupled to the substrate, and the pump and probe beams irradiate aregion including the metal. The metal is detected from the thermalresponse of the metal, which typically is characteristic of the metalcomposition, the shape of the metal, and a pump modulation frequency.The modulation frequency is selected, in response to the thermalresponse, so as to generate an image of the metal at the higherresolution described above.

In some embodiments, an effective wavelength for the higher resolutionis given by the equation:

$\frac{1}{\lambda_{eff}} = {\frac{1}{\lambda_{pump}} + \frac{1}{\lambda_{probe}}}$

where λ_(eff) is the effective wavelength, λ_(pump) is the pumpwavelength, and λ_(probe) is the probe wavelength.

Alternatively, the effective wavelength for the higher resolution isgiven by the equation:

$\frac{1}{\lambda_{eff}^{2}} = {\frac{1}{\lambda_{pump}^{2}} + \frac{1}{\lambda_{probe}^{2}}}$

In some embodiments, the modulation frequency may be varied. Thevariation may enable a depth below the surface of the substrate to bechosen at which inspection is to be performed. Alternatively, if theregion inspected comprises a metal, the variation may enable a thermalcapacity of the metal to be measured.

In some embodiments, the roles of the first and second lasers may beswitched, so that the inspection apparatus is able to function in twomodes. Operation in the first mode is substantially as described above.In the second mode, the second laser operates as the pump laser and ismodulated, whereas the first laser is unmodulated and operates as theprobe laser. The optical system is typically configured so that theoptical properties of the paths followed by both beams, and thedetection systems for both beams, are substantially similar. Returningprobe beam radiation from both modes may be compared, and generates moreinformation of the irradiated region than either mode alone.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken, together withthe drawings, a brief description of which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram illustrating a wafer inspectionapparatus, according to an embodiment of the present invention;

FIG. 1B is a schematic diagram illustrating a wafer inspectionapparatus, according to an alternative embodiment of the presentinvention;

FIG. 2 shows schematic intensity vs. time graphs for the apparatus,according to an embodiment of the present invention;

FIG. 3 shows schematic graphs of effective wave length vs. pump wavelength, according to an embodiment of the present invention;

FIG. 4 shows schematic graphs of effective wave length vs. pump wavelength, according to a disclosed embodiment of the present invention;

FIG. 5A is a chart used to display images, and FIG. 5B is a flowchartillustrating how the chart is generated, according to an embodiment ofthe present invention;

FIG. 6 shows schematic illustrations of a set of images generated by theapparatus of FIG. 1, according to an embodiment of the presentinvention;

FIG. 7A is a schematic diagram of gratings imaged by the apparatus, andFIG. 7B shows schematic graphs of image contrast vs. pitch for thegratings, according to an embodiment of the present invention;

FIG. 8 is a schematic graph of temperature vs. distance, according to anembodiment of the present invention;

FIG. 9 is a schematic graph of intensity vs. modulation frequency,according to an embodiment of the present invention; and

FIG. 10 is a schematic graph of intensity vs. modulation frequency,according to an alternative embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Reference is now made to FIG. 1A, which is a schematic diagramillustrating a wafer inspection apparatus 20, according to an embodimentof the present invention. Apparatus 20 is used to inspect features onand/or close to a surface of an object 70, herein by way of exampleassumed to be a wafer produced in a semiconductor fabrication process,and the description hereinbelow is directed to such a use. However, itwill be appreciated that apparatus 20 may be used to inspect features onand/or close to a surface of materials produced in other processes.Apparatus 20 is typically incorporated into an inspection tool whereinwafer 70 is being inspected, although herein the inspection tool andelements associated with the inspection tool are not shown for reasonsof clarity.

As is described in more detail below, the inspection process used byapparatus 20 is based on irradiating a top surface 69 of wafer 70 withradiation, herein termed pump radiation, that interacts with the wafer.Hereinbelow the pump radiation is assumed to be intensity modulated at amodulation frequency. Typically, surface 69 comprises a plurality ofentities having different characteristics. For example, the surface maybe composed of metals such as copper, aluminum, gold, or titanium,semiconductors such as monocrystalline silicon, polycrystalline silicon,or silicon which has been doped with different elements and/or atdifferent concentrations, and dielectrics such as silicon dioxide orsilicon nitride. The surface is formed according to generally standardprocedures in a fabrication facility, so that an image of the surfacemay be correlated with an expected arrangement of the differententities.

The wafer is inspected by irradiating the wafer with probe radiation,and receiving and analyzing the probe radiation that returns from thewafer. The returning probe radiation comprises radiation which ismodulated at the modulation frequency. The modulated radiation isextracted from the returning radiation, typically using a lock-inamplifier detection system, although any other convenient system may beused. The analysis of the extracted radiation may then be performed by aprocessor 36 using software stored in a memory 41. The processor and thesoftware also operate apparatus 20 and its component elements. A userinterface 38 coupled to the processor allows an operator of apparatus 20to control the operation of the processor, and to see the results.

User interface 38 comprises a color monitor 39, which displays theresults generated by the processor to the operator. As is described inmore detail below, the results may be displayed on display 39 indiffering colors, the colors being selected to present different aspectsof the results, using a lookup table 43 which is stored in memory 41.The lookup table is described in more detail with reference to FIG. 5Bbelow. Presenting the results in different colors enhances the abilityof the operator to register significant aspects of the processed image,although it will be appreciated that any other method of viewing theinformation is plausible.

Some elements of apparatus 20 are assumed, by way of example, to bepositioned relative to a set of orthogonal x, y, z coordinate axes,where the x and y axes are in the plane of the paper and the z axis isout of the plane of the paper. For example, surface 69 is assumed to liein an xz plane. It will be appreciated, however, that the assumptionthat some elements of apparatus 20 are positioned relative to aparticular axis or set of axes is purely for the purposes of clarity inthe following description, and that the elements may be positioned inany convenient orientation.

Apparatus 20 comprises a first source 22, typically comprising a diodelaser 24, which transmits a first beam 34. En an embodiment of thepresent invention, the wavelength of the first source is selected to bein a range 400 nm-550 nm. However, it will be appreciated that thewavelength may be selected to have any other beneficial value. Unlessindicated otherwise below, first source 22, first beam 34, and thewavelength of the first source may also be referred to respectively aspump source 22, pump beam 34, and the first wavelength or the pumpwavelength.

The output from source 22 is intensity modulated at a frequency f.Typically the modulation of beam 34 may be accomplished by a modulationunit 37 that is driven by processor 36, which determines the frequency fand a percentage p by which the intensity is modulated. Typically f isof the order of 1 MHz, and the modulation amplitude p is set to be ofthe order of 100%, corresponding to unit 37 operating source 22 in anon-off mode. Hereinbelow, except as otherwise stated, processor 36 isassumed to maintain frequency fat a substantially constant value.

Beam 34 passes via optics 40 through a first beam splitter 42, whichdiverts a portion of the energy of the beam via a focusing lens 44 to apump reference detector 46. Processor 36 receives respective signalsgenerated by the detector, and may use the signals for normalization ofreceived image signals. Beam 34 passes through a half-wave plate 48 andoptionally a quarter-wave plate 50, acting to control the beampolarization, to a beam combiner 52. Combiner 52 is configured totransmit the wavelength of beam 34, and to reflect the wavelength ofbeams from a probe beam generator 94, described below. The path followedby beam 34 is shown schematically as a line 33.

The components generating beam 34, i.e., those numbered 22, 24, 40, 42,44, 46, 48, and 50, comprise a pump beam generator 54 which generatespump beam 34.

Probe beam generator 94 comprises a second source 62, comprising a laser64, typically a diode laser, which projects a second beam 74. The laseris selected to generate a wavelength, which, in one embodiment of thepresent invention, is selected to be in a range 600 nm-800 nm. The pathfollowed by beam 74 is shown schematically as a broken line 73. Unlessindicated otherwise below, second source 62, second beam 74, and thewavelength of the second source may be referred to respectively as probesource 62, probe beam 74, and the second wavelength or the probewavelength.

Generator 94 further comprises a collimating lens 80, a beam splitter82, a focusing lens 84, a probe reference detector 86, a half-wave plate88, and a quarter-wave plate 90, which are configured in substantiallythe same relative arrangement and perform substantially the samefunctions as components 40, 42, 44, 46, 48, and 50 respectively. Thus,generator 94 transmits a parallel probe beam 74 to beam combiner 52.However, unlike pump beam generator 54, processor 36 does not intensitymodulate probe beam 74, and may use the signals derived from detector 86to normalize the reflected signals so as to eliminate probe laser noise.The beams output from combiner 52, comprising parallel beam 34 andparallel beam 74, are herein termed beam pair 102. Typically beam pair102 comprises coaxial beams, although the pair may be non-coaxial. Forthe purposes of clarity, the beams are shown as separated in FIG. 1.

Pair 102 traverses a set of relay optics 104, a beam splitter 106, andan aperture 108 in a mirror 110, to a scanning mirror 112 which isconfigured to scan the pair in two dimensions on surface 69. Optics 104images the beam pair onto mirror 112; the functions of beam splitter 106and mirror 110 are described below.

Scanning mirror 112 reflects beam pair 102, via a relay and focusingoptical system 114, to top surface 69 of wafer 70. Wafer 70 is mountedand supported on a stage 116, typically a motion stage which is able toalter the position of wafer in the x, y and z directions. Optical system114 is configured to focus beam pair 102 to a pump spot 118 from thepump beam and to a probe spot 119 from the probe beam, both spotsirradiating surface 69. Spots 118 and 119 are generally circular, andare typically configured to be substantially concentric on surface 69.System 114 is typically configured so that a size of spots 118 and 119,the size being measured as a diameter of the spot at the half-powerpoints of the spot, is approximately equal to the Abbe limit of thewavelength generating the spot.

During operation of apparatus 20, scanning mirror 112 scans beam pair102 so that spots 118 and 119 move over surface 69 at a scan rate thatis determined by processor 36. Scanning mirror 112 is operated by amotion stage 120, the mirror and stage 120, together with stage 116,being configured so that spots 118 and 119 may be positioned atsubstantially any location on surface 69. It will be appreciated thatapparatus 20 may comprise other scanning units, known in the art, forscanning the spots over surface 69. For example, mirror 112 may comprisea single plane mirror, and/or a polygonal mirror formed from a number ofdifferent plane mirrors. Such mirrors may be mechanically orelectro-mechanically scanned. Alternatively or additionally, scanning ofthe spots may be accomplished using an acousto-optic deflector, or byother means known in the art, such as mechanical movement of stage 116.

In some embodiments of the present invention, pump beam 34 and/or probebeam 74 are linearly polarized. Alternatively or additionally one ormore of the beams are circularly or elliptically polarized. Thepolarizations are produced by the half and quarter wave plates which thebeams traverse.

As described above, beam pair 102 generates spots 118 and 119 at thesurface 69 of wafer 70, and the beam pair interacts with the surface togenerate reflected and/or scattered radiation from the locations on thesurface irradiated by the spots. The reflected and/or scatteredradiation is hereinbelow referred to as returning radiation, and exceptwhere otherwise stated, the returning radiation is assumed by way ofexample to be from location 122. The elements of apparatus 20 areconfigured so that optical system 114 collects at least a portion of thereturning radiation, and together with mirror 112 directs the collectedportion to initially traverse substantially the same path as theincoming beam pair. Thus the collected portion of the returningradiation returns to mirror 110 and beam splitter 106.

The returning radiation comprises, specular and scattered radiation fromlocation 122. The specular radiation, also known as bright field (BF)radiation, passes through aperture 108 to beam splitter 106. Splitter106 is configured to be substantially transparent to pump beamradiation, and to partially reflect probe beam radiation. Thus, specularprobe beam radiation is reflected by splitter 106, via a narrow bandtransmission filter 124 and focusing optics 126, to an imaging detector128. Filter 124 typically transmits probe wavelengths and is opaque topump wavelengths. Signals from detector 128 are transferred to processor36.

The scattered radiation from location 122, also known as gray field (GF)or dark field (DF) radiation, is reflected by mirror 110, via a narrowband transmission filter 130, which is generally similar to filter 124,and focusing optics 132, to an imaging detector 134. Detector 134detects scattered probe returning radiation, and signals from thedetector are transferred to processor 36.

Typically, detector 128 and detector 134 comprise charge coupled devices(CCDs,) PIN diodes, photodiodes, or photo-multiplier tubes. Eachdetector is also assumed to comprise a respective analog-to-digitalconverter (ADC), which digitizes the signal formed on the detector, andtransfers the digitized signals to processor 36. A sample time of eachADC, taken together with the scan rate of spots 118 and 119 describedabove, determines the effective pixel size used by apparatus 20.Processor 36 sets the scan rate of the spots, and/or the sample time ofthe ADC of detectors 128 and 134, to generate a required pixel size.Alternatively, detector 128 and/or detector 134 may comprise an array ofsub-detectors.

As stated above, processor 36 controls the adjustments of elements ofthe apparatus such as mirror 112, and processor 36 is in turn controlledby an operator of apparatus 20 via user interface 38. Processor 36 alsoreceives the output from imaging detectors 128 and 134, and processesthe outputs to provide results of the inspection of wafer 70. Detectors128, 134, and processor 36 act as a receiving unit 140 for apparatus 20that generates the results. The results may be accessed by the apparatusoperator via interface 38.

FIG. 1B is a schematic diagram illustrating a wafer inspection apparatus30, according to an alternative embodiment of the present invention.Apart from the differences described below, the operation of apparatus30 is generally similar to that of apparatus 20 (FIG. 1A), and elementsindicated by the same reference numerals in apparatus 20 and apparatus30 are generally similar in construction and in operation. Apparatus 30comprises a switch 21 which may couple the modulation from unit 37 tosource 22 or to source 62. Apparatus 30 comprises a first set of narrowband transmission filters 130A and 124A, which are substantially similarto filters 130 and 124, transmitting the second wavelength. Apparatus 30also comprises a second set of narrow band transmission filters 130B and1248, which transmit the first wavelength.

Apparatus 30 is configured to operate in two modes, in contrast toapparatus 20 which operates in one mode. In apparatus 30, operation in afirst mode is substantially as described above for apparatus 20, whereinmodulation unit 37 modulates first source 22, which acts as a pumpsource. In the first mode second source 62 is not modulated andfunctions as a probe source. Operation in the first mode isschematically illustrated by a first position 23 of an arm of switch 21shown in FIG. 1. In the first mode, filters 130A and 124A are positionedin front of detectors 134 and 128, and perform substantially similarfunctions as filters 130 and 124 of apparatus 20, transmitting at thesecond wavelength.

In a second mode of operation, the roles of first source 22 and secondsource 62 are reversed, so that second source 62 acts as a pump source,and first source 22 acts as a probe source. Second source 62 ismodulated by unit 37, and first source 22 is unmodulated. Operation inthe second mode is schematically illustrated by a second position 25 ofthe arm of switch 21. In the second mode, filters 130B and 124B arepositioned in front of detectors 134 and 128, in place of filters 130Aand 124A. Filters 130B and 124B perform substantially similar functionsas filters 130A and 124A, except that filters 130B and 1248 transmit atthe first wavelength, which in the second mode is the wavelength of theprobe source.

In the second mode of operation, generator 94 becomes a pump beamgenerator, and generator 54 becomes a probe beam generator.

In apparatus 30, beams 34 and 74 follow different paths, although someof the elements in the paths are common. Apparatus 30 is typicallyconfigured so that the optical parameters of each of the paths match.The optical parameters include, but are not limited to, an overallnumerical aperture of each of the paths, and numerical apertures ofindividual optical elements in the paths.

Apparatus 30 is typically operated by scanning surface 69 in the firstand the second mode, and analyzing the returning probe beam signalsreceived in both modes. It will be understood that switching between thetwo modes may be performed by processor 36, with substantially no changein the architecture of apparatus 30.

The description below, except where indicated otherwise, assumes thatapparatus 20 is operative. Those having ordinary skill in the art willbe able to apply the description, mutatis mutandis, to the operation ofapparatus 30.

FIG. 2 shows schematic intensity vs. time graphs for apparatus 20,according to an embodiment of the present invention. A first graph 150illustrates the intensity modulated output from pump source 22, asmeasured by detector 46 (FIG. 1). By way of example, herein themodulation is assumed to be an on-off modulation generating a squarewave, with approximately equal times for the on and off periods.However, any other convenient intensity modulation, such as a sine wavemodulation, may be applied to the pump source. A lower graph 152schematically illustrates, on a different intensity scale from that ofthe pump source, the intensity modulated signal generated at detector128 and/or detector 134 due to returning probe radiation. Typically, theAC component of the returning probe radiation is of the order of 10⁻³times the DC component of the returning probe radiation.

Pump spot 118 is intensity modulated as a consequence of the intensitymodulation of the pump radiation. The modulation of the pump spot causessubstantially synchronous changes in characteristics of location 122.The character changes of location 122 modulate the probe radiationirradiating the location, so that the returning probe radiation,illustrated in graph 152, is also modulated in synchronization with thepump radiation. By way of example, the intensity for graph 152 isassumed to comprise the sum of the signals on detectors 128 and 134, anda mean level 154 of the graph is assumed to be the intensity of thereturning probe radiation.

However, as shown in graphs 150 and 152, there is typically a phasedifference, also herein termed a phase shift, between the pump radiationand the returning probe radiation. The phase shift depends on thecharacteristics of location 122. For example, if location 122 comprisesmetal, the phase shift is typically close to zero. If location 122comprises polycrystalline silicon, the returning probe radiation istypically delayed by approximately 45° compared to the pump radiation.The phase shift values given here are for f of the order of 1 MHz, andtypically depend on the value of f, and are also somewhat influenced bythe geometry of the feature under inspection. As is explained in moredetail below, embodiments of the present invention use the shifts inphase of returning probe radiation to identify different materials onsurface 69.

FIG. 3 shows schematic graphs of effective wave length vs. pump wavelength, according to an embodiment of the present invention. Theresolution of an optical system is a function of an effective wavelengthoperating in the system, and the resolution may be quantified in termsof an Abbe limit (d), which is a spacing of lines on an object beinginspected that can just be resolved by the system. The Abbe limit isequal to about half the diameter of a spot that may be focused onto theobject. For a single wavelength system, having an effective numericalaperture (NA_(eff))≈1:

$\begin{matrix}{{s \approx {\lambda\mspace{14mu}{and}\mspace{14mu} d}} = {\frac{s}{2} \approx \frac{\lambda}{2}}} & (1)\end{matrix}$

where:

-   -   s is the diameter of the spot,    -   λ is the wavelength of the radiation forming the spot, and    -   d is the Abbe limit.

Equation (1) applies for a system such as apparatus 20 if NA_(eff)≈1 andif only pump irradiation, or if only probe irradiation, is used. In thiscase, from equation (1), if surface 69 is irradiated with only pump beam34, at a wavelength λ_(pump)=440 nm, the Abbe limit is approximately 220nm, so that the resolution of the system at this wavelength isapproximately 220 nm. If surface 69 is irradiated with only probe beam74, at a wavelength λ_(probe)=635 nm, the Abbe limit is approximately318 nm; so that the resolution in this case is approximately 318 nm.

As described above, surface 69 is irradiated by both the pump and theprobe beam simultaneously. The pump radiation from pump source 22interacts with the material, at location 122, that is irradiated. Iflocation 122 comprises a metal or a dielectric, the main interaction istypically heating of the material. If location 122 comprises asemiconductor, the main interaction is typically excitation of carriersin the material.

The probe radiation returning to detectors 128 and 134 is effectively amultiplication of the point spread function (PSF) of probe spot 119 withthe response of the material to the irradiation of the pump spot 118.When provisions are made (by controlling the pump modulation) so thatthe material reaction to the pump is similar to the PSF of pump spot118, there is effectively a multiplication of pump and probe spots. Themultiplication causes an effective wavelength λ_(eff) of imagesgenerated by the pump-probe irradiation to be less than the pumpwavelength λ_(pump), and also to be less than the probe wavelengthλ_(probe), and a theoretical relationship between λ_(eff), λ_(pump), andλ_(probe) is given by equation (2):

$\begin{matrix}{\frac{1}{\lambda_{eff}} = {\frac{1}{\lambda_{pump}} + \frac{1}{\lambda_{probe}}}} & (2)\end{matrix}$

The derivation of equation (2) will be apparent to persons havingordinary skill in the optical arts. Equation (2) may be rearranged toequation (3):

$\begin{matrix}{\lambda_{eff} = \frac{\lambda_{probe} \cdot \lambda_{pump}}{( {\lambda_{pump} + \lambda_{probe}} )}} & (3)\end{matrix}$

Graphs 200, 202, 204, and 206 illustrate the relationship of equations(2) and (3) for four probe wavelengths: 635 nm, 532 nm, 440 nm, and 266nm. For example, the equations and graph 206 show that for a value ofλ_(pump)=440 nm and λ_(probe)=635 nm, the effective wavelength λ_(eff)of images generated by apparatus 20, if NA_(eff)≈1, is 260 nm. In thiscase, equation (1) gives the Abbe limit as approximately 130 nm, so thatthe resolution of apparatus 20 using both the pump and the probe beamssimultaneously is approximately 130 nm. Processor 36 and detectors 134and 128 are thus advantageously configured so that the respective ADCsof the detectors are sampled at a sample time that generates an imagepixel size of approximately 130 nm.

On consideration of the resolutions above, it will be appreciated thatthe resolution of apparatus 20 operating both the probe and the pumpbeams simultaneously is better than the resolution of the apparatusoperating with only the pump beam or only the probe beam.

FIG. 4 shows schematic graphs of effective wave length vs. pump wavelength, according to a disclosed embodiment of the present invention. Inthe disclosed embodiment described herein, the inventors determined thatNA_(eff)≈0.7. In this case, equation (1) becomes:

$\begin{matrix}{{s \approx {\frac{\lambda}{{NA}_{eff}}\mspace{14mu}{and}\mspace{14mu} d}} = {\frac{s}{2} \approx \frac{\lambda}{2 \cdot {NA}_{eff}}}} & (4)\end{matrix}$

The inventors have found that for the disclosed embodiment arelationship between λ_(eff), λ_(pump), and λ_(probe) is given byequation (5):

$\begin{matrix}{\frac{1}{\lambda_{eff}^{2}} = {\frac{1}{\lambda_{pump}^{2}} + \frac{1}{\lambda_{probe}^{2}}}} & (5)\end{matrix}$

Equation (5) may be rearranged to equation (6):

$\begin{matrix}{\lambda_{eff} = \frac{\lambda_{probe} \cdot \lambda_{pump}}{( {\lambda_{pump}^{2} + \lambda_{probe}^{2}} )^{1/2}}} & (6)\end{matrix}$

Graphs 250, 252, 254, and 256 illustrate the relationship of equations(5) and (6) for four probe wavelengths: 635 nm, 532 nm, 440 nm, and 266nm. For example, the equations and graph 256 show that for a value ofλ_(pump)=440 nm and λ_(probe)=635 nm, the effective wavelength λ_(eff)of images generated by apparatus 20 is 362 nm. For the disclosedembodiment, the Abbe limits, from equation (4) are respectivelyapproximately 310 nm, 450 nm, and 260 nm, for the pump beam operatingalone, for the probe beam operating alone, and for the pump and probebeams operating simultaneously.

As for the embodiment described above with respect to FIG. 3, theresolution of apparatus 20, for NA_(eff)≈0.7, for operating both theprobe and the pump beams simultaneously is better than the resolution ofthe apparatus operating with only the pump beam or only the probe beam.

FIG. 5A illustrates a color chart 300 used to display images, and FIG.5B is a flowchart 320 illustrating how chart 300 is generated, accordingto an embodiment of the present invention. In FIG. 5A, differentshadings represent different colors. Typically there are significantlymore colors in chart 300 than are shown in the figure. Chart 300 gives acolor that is used on color monitor 39 (FIG. 1) to register the phasedifferences of returning probe radiation compared with the pumpradiation. In an embodiment of the present invention, the phasedifferences correspond to characteristic differences on surface 69, asdescribed above with reference to FIG. 2.

In some embodiments of the present invention, to generate chart 300, anoperator of apparatus 20 performs the steps of flowchart 320. In a firststep 322, the operator selects location 122 to have a known material,for example, location 122 may be selected to comprise n-doped silicon.Location 122 is then irradiated by the pump and probe radiations.

In a second step 324 a phase shift of the returning probe radiation,compared with the incident pump radiation, is measured by processor 36using signals from detector 46, detector 134, and/or detector 128.

In a third step 326, processor 36 stores in memory 41 the phase shiftand an identity of the material selected in step 322.

Typically, the operator iterates steps 322, 324, and 326 for differentknown materials on surface 69, as indicated by a line 328 of theflowchart. After the iterations, flowchart 320 ends, and the differentpairs of values of material identity and measured phase difference arestored in lookup table 43.

It will be understood that flowchart 320 is one example of a method forcorrelating phase shifts of returning probe radiation with a compositionor characteristic of the irradiated material. Other methods forgenerating the correlation of lookup table 43, such as scanning a regionof surface 69 having known constituents, will be familiar to thosehaving ordinary skill in the art, and are assumed to be included in thescope of the present invention.

Chart 300 shows the materials polycrystalline silicon (POLY), a metal,and p- and n-type doped silicon, and the respective phase shiftsgenerated by the materials, as recorded in lookup table 43. Thedifferences in phase shift are typically caused by the differences inmaterial, and by the differences in time for an irradiated region toreach a quasi-steady state. Processor 36 assigns colors shown in thechart to the phase shifts, so that lookup table 43 shows equivalencesbetween materials, generated phase shifts, and assigned colors. Table Ibelow shows entries in lookup table 43, corresponding to FIG. 5A.

TABLE I Material Generated Phase Shift Assigned Color PolycrystallineSilicon  −45° Purple Metal   0° White p-doped silicon, n-doped +135°Green silicon

The embodiments described above have described how apparatus 20generates images differentiating between materials having differentcharacteristics. The description above is by way of example, so that itwill be understood that materials having other types of differentcharacteristics may be differentiated. For example, silicon doped with alow level of cobalt may be differentiated from silicon doped with a highlevel of cobalt because the two types of silicon have different bandgaps which generate different levels of excess carriers. The differentexcess carrier levels may generate different phase shifts in graph 152(FIG. 2).

FIG. 6 shows schematic illustrations of a set of images generated byapparatus 20, according to the disclosed embodiment of the presentinvention described above with reference to FIG. 4. For all the imagesin FIG. 6, the scan rate of spots 118 and 119, and the sample time ofthe ADCs of detectors 134 and 128, are set so that a pixel size of theimage corresponds with the effective wavelength λ_(eff) given byequations (5) and (6). The images are generated with a pump beam havinga wavelength λ_(pump)=440 nm, a probe beam having a wavelengthλ_(probe)=635 nm, and a pixel size of approximately 260 nm,corresponding with the Abbe limit of the disclosed embodiment operatingwith both the pump and the probe beams. The images are of a region ofsurface 69 having a repetitive pattern.

A first gray-scale image 350 is produced by detectors 134 and 128 usingonly probe beam 74. A second gray-scale image 352 is generated usingintensities measured by the detectors when both probe beam 74 and pumpbeam 34 are operative. Comparison of image 350 with image 352 shows thatin image 352 there is significantly more detail resolved, illustratingthe gain in resolution obtained by the disclosed embodiment.

In a color image 370, phase information from the detectors,corresponding to that described above for FIG. 5A, is incorporated intoimage 352. By way of example, image 370 has two colors, purple andgreen, derived from chart 300, applied to the image. In image 370,sections of the image in line with arrows P are purple, and sections ofthe image in line with arrows G are green. For clarity in image 370,only parts of the image have been labeled with colors P, G. It isassumed that a process generally similar to flowchart 320 has beenimplemented to generate the chart of Table I. The phase information hasbeen translated into colors according to chart 300 and Table I above, sothat purple represents polycrystalline silicon, and green represents p-or n-doped silicon.

Image 370 combines the high image resolution provided by apparatus 20,the phase shift measured by the apparatus, and the application of colorto the image. The combination provides an image to an operator ofapparatus 20 which has significantly more information than image 350.Furthermore, the application of color to the image provides theinformation to the operator in an easily assimilated form. It will beappreciated that such a presentation enables an operator of apparatus 20to quickly and accurately locate the position of a feature, such as adefect, on surface 69, as well as to identify the composition of thefeature. Images 354 and 372 exemplify such a defect.

Gray-scale image 354 and color image 372 are of a region of the patternhaving a defect. Images 354 and 372 are generated substantially asdescribed above for images 352 and 370 respectively, and for clarity areshown in FIG. 6 as enlarged. In image 372 colors of regions in line witharrow P are purple, and regions labeled with a suffix G are green.Inspection of images 354 and 372 shows that detail of the defect is wellresolved. In addition, image 372 shows that defect regions 374G and 376Ghave the anomalous presence of doped silicon, and that a defect region378 has the anomalous absence of polycrystalline silicon.

FIG. 7A is a schematic diagram of gratings imaged by apparatus 20, andFIG. 7B shows schematic graphs of image contrast vs. pitch for thegratings, according to an embodiment of the present invention. Thegraphs are experimental graphs produced using the disclosed embodimentof apparatus 20 described above, for ten different gratings at location122. The gratings are parallel polycrystalline silicon lines plated on adoped silicon layer, so that there are parallel doped silicon spacesbetween the lines. The widths of the lines and the spaces between thelines are substantially equal. The graphs show measured image contrastsfor differently pitched sets of lines.

The plated lines have pitches of approximately 272 nm, 300 nm, 335 nm,370 nm, 400 nm, 450 nm, 500 nm, 600 nm, 700 nm, and 800 nm. A diagram400 illustrates the smallest pitch grating of 272 nm; a diagram 402illustrates the largest pitch grating of 800 nm. In FIG. 7B a graph 500is for apparatus 20 operating with a pump wavelength λ_(pump)=440 nm anda probe wavelength λ_(probe)=635 nm simultaneously. Graph 502 is for theapparatus operating only with the pump beam, and graph 504 is for theapparatus operating only with the probe beam. FIG. 7B also shows a lowerleft section of graphs 500, 502, and 504 on a magnified scale.

Inspection of the graphs demonstrates that the resolution of thecombined pump-probe system is significantly greater than either the pumpbeam system or the probe beam system alone. Thus, graph 500 shows thatthe combined pump-probe system cuts off at approximately 270 nm, whereasusing the probe alone, at a wavelength of 440 nm, the cut off isapproximately 350 nm. The inventors measured the cut off for the probealone, operating at a wavelength of 635 nm, to be approximately 470 nm.The combined pump-probe beam cut-off of approximately 270 nm correspondswith the predicted cut-off of 260 nm given by equations (4) and (5).

The graphs of FIG. 7B are for materials comprising different types ofsemiconductors. In this case, the pump radiation typically causeschanges in the returning probe radiation because of different levels ofexcess carriers in the different types of semiconductors.

The inventors have found generally similar effects to those illustratedin the graphs of FIG. 7B for sets of parallel metal lines plated on adielectric or on a semiconductor surface. In the case of metals, thechanges in the returning probe radiation are caused by the temperaturechange induced in the metal by the modulation of the pump radiation,which in turn is a function of the thermal response, i.e., the diffusiontime scale, of the metal. The thermal response of the metal depends onthe physical dimensions of the metal, for example, a thin parallel metalstrip has a different response from a thin metal plane. The thermalresponse also depends on the specific heat capacity of the metal, theresponse changing inversely with the specific heat capacity. Inaddition, the temperature change induced into the metal depends on themodulation frequency of the pump radiation. In general, for a regionthat includes a metal, to achieve resolutions in a given direction ofthe region that are comparable to those described above forsemiconductors, the modulation frequency is typically higher than thediffusion time scale in the direction, so that there is substantially nodiffusion of heat from the pump spot in that direction.

The increased resolution of the effective reduced wavelength λ_(eff) mayalso be applied to improving the resolution of metal features on surface69 other than those described above. For example, substantially similarimproved resolution images may be formed for temperature variations inmetals, such as in the example described below with reference to FIG. 8.

FIG. 8 is a schematic graph of temperature vs. distance, according to anembodiment of the present invention. The graph shows the temperaturevariation, measured by detectors 128 and/or 134 in the disclosedembodiment described above, along a copper line on a silicon substrate.The copper line has periodic width variations along the line, and athermal current generated by the pump beam flows in the line, causingthe temperature of the line to vary. The high temperature sectionscorrespond to narrow sections of the line, the low temperature sectionscorrespond to wide sections of the line. The graph illustrates thevariation in temperature as measured at the improved resolution ofλ_(eff).

FIG. 9 is a schematic graph of intensity vs. modulation frequency,according to an embodiment of the present invention. The graph gives theintensity determined by detectors 134 and/or 128 as the modulationfrequency f is varied, when surface 69 comprises a metal. In the case ofa metal, the level of carriers is not a function of the pump beamintensity or wavelength, and is approximately constant. The intensitygiven by detectors 134/128 mainly varies only according to thetemperature of the metal, as is exemplified above by the graph of FIG.8.

For low frequencies of modulation of the pump beam, the intensity of thereturning probe radiation is quasi-steady, since the diffusion of heatfrom the metal keeps pace with the changes of heat flux due to themodulation of the pump beam. As the modulation frequency increases, thediffusion of heat begins to be unable to follow the changes of heatflux, leading to the intensity of the returning probe beam reducingapproximately exponentially, at a rate that is approximately directlyproportional to a thermal capacity of the irradiated metal. Thus, thechange of modulation frequency Δf required to decrease the returningprobe beam intensity by a factor of e in the intensity gives a measureof the thermal capacity Q of the metal irradiated, i.e.,Q≈k₁·Δf  (7)

-   -   where: k₁ is a constant which may be determined by calibration        of the apparatus.

Thus, apparatus 20 may be used to measure the thermal capacity Q of theirradiated metal. A thermal capacity measurement can be beneficial sinceit is closely correlated to the feature geometrical dimensions, such asa thickness of the feature.

FIG. 10 is a schematic graph of intensity vs. modulation frequency,according to an embodiment of the present invention. The graph gives theintensity determined by detectors 134 and/or 128 as the modulationfrequency f is varied, when surface 69 comprises a semiconductor. In thesemiconductor, the level of excess carriers is a function of the pumpbeam intensity and of the pump wavelength. At relatively low frequenciesof pump modulation, the excess carriers diffuse from the region ofirradiation on surface 69, typically to depths of the order of 10 μm,before the concentration of the carriers effectively becomes zero by aprocess of recombination. Thus, in a region 600 of the graph, atrelatively low pump beam modulation frequencies below of the order of 1MHz, the depth of the excess carriers is effectively limited by thediffusion depth of the carriers.

At frequencies higher than of the order of 1 GHz, in a region 604 of thegraph, the depth of the excess carriers is effectively limited by thepenetration depth of the pump beam. The penetration depth of the pumpbeam depends on the wavelength of the pump beam and is of the order of0.1 μm for crystalline silicon.

However, in an intermediate region 602 of the graph, at pump beammodulation frequencies between of the order of 1 MHz and of the order of1 GHz, the effective depth of the excess carriers may be varied byadjusting the pump beam modulation frequency.

In the intermediate region, if the modulation frequency increases, theincreased frequency effectively reduces the depth to which the excesscarriers penetrate, as illustrated by the graph. In a portion of region602 an effective depth D of the carriers is approximately inverselyproportional to the modulation frequency f, i.e.,

$\begin{matrix}{D \approx {k_{2} \cdot \frac{1}{f}}} & (8)\end{matrix}$

-   -   where: k₂ is a constant which may be determined by calibration        of the apparatus.

Furthermore, a relation between D and f for the whole of region 602 maybe determined without undue experimentation. Thus, in a limited range offrequencies, exemplified by region 602 of the graph of FIG. 10,apparatus 20 may be used to adjust an effective depth of the excesscarriers generated in an irradiated semiconductor. The adjustmentenables an operator of apparatus 20 to inspect a specimen such as wafer70 at a chosen depth beneath surface 69. The control over the depth isof importance for looking for sub surface defects on the one hand, in alow f regime, and for blocking out a sub layer signal on the other hand,in a high f regime.

Returning to FIG. 1B, operating apparatus 30 in either the first mode orthe second mode may provide all the information determined for surface69, as described above with reference to FIGS. 2-10. In addition,processor 36 may switch between the two modes of apparatus 30 accordingto the physical responsiveness of the materials under inspection, anduse measurements of both modes. The complementary measurements generatedby alternating pump and probe lasers can yield more informationregarding surface 69 than is available by just operating in one mode.

The complementary measurements may also be beneficial as some materialsmay have a stronger response to excitation or pumping in the wavelengthof source 22 and probing with source 62, as opposed to the oppositecase. For example, operating in a first mode that pumps at a wavelengthbelow the energy band gap of a semiconductor in surface 69 may notgenerate a strong signal from the semiconductor, but may generate astrong signal from another element of the surface. Operating in a secondmode that pumps at a wavelength above the energy band gap of thesemiconductor typically does generate a strong signal from thesemiconductor, but may not generate a strong signal from the otherelement.

It will be appreciated that the embodiments described above are cited byway of example, and that the present invention is not limited to whathas been particularly shown and described hereinabove. Rather, the scopeof the present invention includes both combinations and subcombinationsof the various features described hereinabove, as well as variations andmodifications thereof which would occur to persons skilled in the artupon reading the foregoing description and which are not disclosed inthe prior art.

1. Apparatus for inspecting a region of a substrate, comprising: a firstbeam generator that generates a first beam at a first wavelength λ1; asecond beam generator that generates a second beam at a secondwavelength λ2; a modulation unit operative in a first mode to modulatean intensity of the first beam at a first frequency while an intensityof the second beam is unmodulated, and in a second mode to modulate theintensity of the second beam at a second frequency while the first beamis unmodulated; an optical system which is configured to convey thefirst beam and the second beam to the region; and a detector thatreceives returning radiation from the region generated in response tothe modulation unit operating in the first mode and the second mode, andthat forms an image of the region in response to the returningradiation.
 2. The apparatus according to claim 1, wherein the first beamfollows a first optical path through the optical system and the secondbeam follows a second optical path through the optical system, andwherein the first and second optical paths have matching opticalparameters.
 3. A method for inspecting a region of a substrate,comprising: generating a first beam at a first wavelength λ1; generatinga second beam at a second wavelength λ2; modulating the intensity of thefirst beam at a first frequency in a first mode of operation while anintensity of the second beam is unmodulated, and modulating theintensity of the second beam at a second frequency in a second mode ofoperation while an intensity of the first beam is unmodulated; conveyingthe first beam and the second beam to the region; and receivingreturning radiation from the region generated in response to operationin the first mode and operation in the second mode, and forming an imageof the region in response to the returning radiation.
 4. The methodaccording to claim 3, wherein the first beam follows a first opticalpath through an optical system and the second beam follows a secondoptical path through the optical system, and wherein the first andsecond optical paths have matching, optical parameters.